Photograph by Paul Sutherland, National Geographic
Read Caption

Bumphead parrotfish congregate near a reef.

Photograph by Paul Sutherland, National Geographic

Head-Bashing and Other Mating Secrets of Giant Parrotfish

The giant fish, which help keep coral reefs healthy, have declined due to their popularity as seafood.

Love the beach? You can thank the bumphead parrotfish, Bolbometopon muricatum, a wide-ranging species that eats massive amounts of coral and, well, poops it out as that luxurious, toe-wriggle-worthy sand.

The predators' constant pruning keeps reefs from getting overcrowded and prevents the growth of weedy, invasive corals that can smother it. (See "As Oceans Heat Up, a Race to Save World's Coral Reefs.")

Now, in an effort to help save these valuable fish, a study has unveiled the giants' unusual mating habits—such as head-butting over territory.

Despite its crucial role in keeping coral reefs healthy—and beachgoers happy—the greenish-blue predator is in trouble, in part because it's also a popular dinner entree. (See "Predator Fish Help Coral Reefs Rebound, Study Shows.")

In all of the regions where its Pacific and Indian Ocean habitat overlaps with human activities, the 165-pound (75-kilogram) parrotfish—the world's largest—has been devastated by overfishing. Not only is it considered good eating, the animal's habit of sleeping in large, shallow-water schools makes it an easy target for fishermen. As a result, the International Union for Conservation of Nature lists the bumphead parrotfish as vulnerable to extinction.

But the fish with the funny forehead has found a few coral oases where humans don't reach, including parts of the Great Barrier Reef and Wake Atoll (map), a remote U.S. marine reserve in the western Pacific. (See pictures of protected ocean areas in the U.S.)

To find out more about bumphead parrotfish—and how to protect them—a team of scientists recently ventured to Wake Atoll, a ring of ancient reefs, where they conducted the most comprehensive study yet of the species.

Among their findings, published in late 2014 in Peer J: The fish spawn in big groups under the full moon, bash their heads together (a possible reason for their oddly shaped heads), and mate in organized systems in which males put on shows as females parade by.

The study "gives you this wonderful insight into normal spawning behavior of this species," said David Bellwood, a reef fish ecologist at James Cook University in Townsville, Australia, who has studied bumphead parrotfish and affectionately calls them "bolbos."

Mating With the Moon

At Wake Atoll, Roldan Muñoz, a research fishery biologist at the U.S. National Marine Fisheries Service, and colleagues closely observed several spawning events of bumpheads.

The team had heard reports that the fish's mating is tied to lunar cycles, and sure enough, they found that bolbo groups in the hundreds spawn reliably around the full moon, as well as near the last quarter moon. (Watch video: "Moon 101.")

Why then? It's possible that the tide during those phases of the moon may temporarily push the newly spawned eggs and larvae away from the reef and out of harm's way from predators, said Muñoz, who received funding for a follow-up expedition from National Geographic's Committee for Research and Exploration.

His team also confirmed a theory that the fish mate in what's called a lek system: The males station themselves in small, defined territories on the ocean bottom, bobbing in the current as they wait for schools of between ten and over a hundred females to arrive.

"As females pass through these stationary males' territories, they are vigorously courted by the males with specific short swim bursts in front of the female and temporary color changes," Muñoz said.

When the female decides she likes a male, she approaches him and turns her face white, upon which the male joins her near the surface. The two touch flanks briefly before she releases her eggs and the male fertilizes them. The fish then go their separate ways.


Unexpectedly, the scientists also witnessed larger males vigorously defending their territories by bashing together their formidable domes. Although bolbos are known to smash their heads into coral to make for easier eating, "no one had ever heard that [head-butting] happened with a fish," Muñoz said.

View Images

Bumphead parrotfish swim near Bali, Indonesia.

"The bumping of the heads was a shock," said James Cook University's Bellwood, who wasn't involved in the research. "Those little surprises occur when you get to go to these special locations where [the population is] relatively intact."

Perhaps among the most valuable findings was that the fish were faithful to their spawning locations at Wake Atoll, returning time and time again to the same place to mate.

What's more, all of the action happens in a relatively small area: about 1.7 acres (0.72 hectare). (See National Geographic's coral reef pictures.)

Knowing where fish mate—and how big the area is—is crucial data for conservationists. For instance, bumphead parrotfish spawning sites could be included within the boundaries of marine reserves, Muñoz said.

Unstable Future?

Bellwood agreed that "we have to take heed of these observations and incorporate them into our management plans. We're not acting fast enough to protect this species."

And as bumphead parrotfish decline, so does the quality of the reefs on which they depend. With its extremely tough teeth—which Muñoz calls a "built-in cement grinder"—each bumphead parrotfish can chew and excrete up to five tons of coral a year, Muñoz said.

"We have now effectively taken out one of the big fishes from reef ecosystems. The consequences are largely unknown, but they worry me," Bellwood said.

"Without bolbos, we're opening up the reef to a different kind of dynamic that may be more unstable."

Follow Christine Dell'Amore on Twitter and Google+.